Measuring and controlling of a flow are crucial contents for an integrated gas delivery system. One commonly used device is a mass flow controller (MFC), which controls the introduced gas or liquid tightly. Referring to the FIG. 1, a block diagram of a prior art MFC is shown. The prior art MFC comprises a sensor 11, an electromagnetic valve 12, a sensor driver circuit 14 coupled to the sensor 11 in order to receive a detection signal from that, an electromagnetic valve driver circuit 15 coupled to the electromagnetic valve 12 in order to adjust a flow rate through that, a microprocessor 13 coupled to the sensor driver circuit 14 and the electromagnetic valve driver circuit 15 respectively, and an A/D converter 16. Various parameters of the gas or the liquid introduced into a tubular shunt 17, such as a flow rate, a flow, etc., are sensed by the sensor 11 and converted into an electronic signal to be outputted to the sensor driver circuit 14 for processing. The A/D converter 16 converts an inputted setting signal AS01 into a first digital signal, and converts the processed signal AS02 outputted by the sensor driver circuit 14 into a second digital signal. The microprocessor 13 is coupled to the A/D converter 16 for receiving the first digital signal and the second digital signal to generate a flow rate control signal. Then, the electromagnetic valve driver circuit 15 converts the flow rate control signal outputted by the microprocessor 13 into an analog signal to control the electromagnetic valve 12, so as to control the flow rate of the gas or the liquid. Referring to the FIG. 2, which is a circuit diagram of a closed-loop circuit according to the prior art MFC. In general, the mechanism of the prior art MFC is a control system having a closed-loop circuit. A difference in a detection signal AS02 outputted by a sensor 21 and a setting signal AS01 is calculated by a PID module 22 and converted into a control voltage to control an openness of an electromagnetic valve 23, so as to control a flow rate of a fluid 24 tightly.
In most cases, the analog voltage 0-5V is used to input and output signals according to the prior art MFC. That is to say that, the flow rate setting signal and the flow rate output signal are represented by a voltage value ranged from 0 to 5V. Referring to the FIG. 3, which is the using environment diagram of the prior art MFC. As shown in the FIG. 3, the MFC 1 is connected with a customer terminal 2 by three signal wires which includes a setting signal wire 31, a ground wire 32 and a flow rate signal wire 33. However, the signal will be lost during the transmission in the above-mentioned three signal wires due to the inappropriate arrangement of the ground wire, so as to the inaccuracy of the setting signal and the detection signal. For example, a setting voltage of 5V corresponding to a 100% full scale is provided from the customer terminal. Due to some errors from the ground wire and the system of the MFC, the received setting voltage by the MFC is only 4.95V. Therefore, an error of 0.05V is caused by the using environment, which leads to the inaccuracy of the MFC controlling. Similarly, if a flow rate signal of 5V is outputted from the MFC, the received voltage by the customer terminal is only 4.95V due to some errors from the ground wire and the system of the MFC. Therefore, the same error is caused again.
Accordingly, it is an urgent problem to be solved that how to reduce the error caused by the using environment to improve the accuracy of the MFC controlling and using.